1. Field of the Invention
The present invention relates to heat transfer devices, and more particularly, to a hybrid heat transfer loop system that consists of both capillary driven two-phase flow and actively pumped liquid flow segments.
2. Description of Prior Art
Attaching a heat pipe to a waste heat generating component such as an integrated circuit is a known and successful technique of moving the undesired waste heat away from that electronic component. One portion of the heat pipe is exposed to the heat source and another portion of the heat pipe is exposed to a heat sink, which is at a lower temperature than the heat source. Heat is absorbed from the heat source by evaporation of a liquid-phase working fluid inside the heat pipe at the portion exposed to the heat source (the evaporator). The vapor phase working fluid with its absorbed heat load (latent heat of vaporization) is thermodynamically driven to the portion of the heat pipe exposed to the heat sink, due to the pressure gradient caused by the temperature difference between the heat source and the heat sink. The heat load is transferred by the working fluid to the heat sink, with consequent condensation of the vapor phase working fluid at the portion of the heat pipe exposed to the heat sink (the condenser). Then, driven by the capillary force developed in a capillary structure inside the heat pipe, the condensed working fluid is returned in liquid phase to the evaporator portion of the heat pipe through the capillary structure. The heat pipe is often referred as a passive heat transfer device, due to the fact that its operation is driven by the heat input instead of electrical or mechanical energy.
The capillary structure is typically an elongated wick structure extending for substantially the full length of the heat pipe. Wick structures may include interior longitudinal grooves, roughened interior surfaces, or mesh screens or bonded powders joined to the interior surface of the heat pipe. The capillary limit of a heat pipe is reached when the maximum capillary force available in the wick is not able to overcome the total pressure drop of the liquid and vapor flows inside the heat pipe and the gravity or acceleration effects on the liquid-phase working fluid.
One shortcoming of the heat pipe is that its heat transport capability and heat carrying distance are limited by the capillary limit in the wick structure, which is sensitive to the orientation and magnitude of the gravity. In addition, a high heat flux input at the evaporator presents another significant problem to the heat pipe operation. Heat flux is a commonly used terminology to describe the thermal power density, which has a unit of watt per square centimeter (W/cm2). Modern computer chips typically generate waste heat fluxes on the order of 15 to 50 W/cm2. Next generation power and opto electronics may produce waste heat fluxes in excess of 1,000 W/cm2. The increasing heat flux may cause boiling inside the wick structure in the evaporator portion of the heat pipe, disrupting the returning liquid flow and consequently causing dry-out in the evaporator.
Some prior art approaches to improving the heat carrying capacity of heat pipes include the use of elongated arterial passages along an interior wall of the heat pipe. Examples of such configurations can be found in U.S. Pat. Nos. 4,470,450 and 4,515,207, the disclosures of which are hereby incorporated by reference, in their entirety. In principle, the pressure drop of the liquid flow through the arteries is substantially smaller than through the wick interiors. The reduced liquid pressure drop results in an increase in the liquid flow rate and consequently in heat transport capability and heat carrying distance. However, these arteries are susceptible to boiling at high heat flux conditions, wherein the resultant vapor bubbles can interrupt or block the capillary driven liquid flow through the arteries, resulting in system failures.
Other prior art approaches to improving the heat carrying capacity of heat pipes include the use of separate liquid and vapor transport lines to link the evaporator and condenser. These devices are typically called loop heat pipes or capillary pumped loops, depending on the detailed design configuration. Examples of the loop heat pipe and capillary pumped loop configurations can be found in U.S. Pat. Nos. 6,382,309 and 5,944,092, the disclosures of which are hereby incorporated by reference, in their entirety. The separation of the liquid and vapor lines allows the use of very fine pore wicks in the evaporator portion only, which substantially increases the capillary force in the wick and reduces the pressure drops of the liquid and vapor flows. Typical loop heat pipes and capillary pumped loops can carry significantly more heat over far greater distances than typical heat pipes. In addition, the operations of loop heat pipes and capillary pumped loops have reduced sensitivity to the gravity and acceleration, since the capillary force in the fine pore wicks in the evaporator is far greater than the gravity and acceleration effects.
Loop heat pipes and capillary pumped loops have very limited capabilities of handling high heat flux input. The vapor evaporated from the wick in the evaporator of a loop heat pipe or capillary pumped loop is directly exposed to the heat input before vented to the vapor transport line. The resultant vapor superheating substantially reduces the heat transfer efficiency and limits the heat flux capacity of the loop heat pipe and capillary pumped loop. Typical loop heat pipes and capillary pumped loops cannot handle heat fluxes higher than 20 W/cm2. Some advanced designs have demonstrated close to 100 W/cm2 heat flux capacity in a laboratory setting.
Some prior art approaches to simultaneously obtaining large heat carrying capacity and high heat flux capacity include the use of liquid pumps to replace or supplement the capillary force as the mechanism to return the liquid working fluid to the evaporator. Spray nozzles have also been used to supply liquid to the evaporator and produce a thin film evaporation mode. In either scenario, however, the inability to precisely regulate liquid flow to the evaporator can result in an excess liquid supply to the evaporator, also known as “flooding,” which results in decreased heat transfer performance. Elaborate control schemes involving a complex network of valves and sensors are sometimes used to regulate the liquid supply to the evaporator to prevent flooding. In addition, spray cooling systems are generally prone to clogging in the nozzles, and their performance may be sensitive to gravity and acceleration. Examples of complex valve and sensor control schemes can be found in U.S. Pat. No. 6,349,554.
Liquid pumps have also been combined with arterial heat pipes to improve the heat carrying capacity of heat pipes. For example, the U.S. Pat. Nos. 4,470,450 and 4,898,231 utilize external liquid pumps, which are serially connected between the condensers and evaporators of loop-type heat pipe assemblies. The systems utilize a plurality of discrete valves and sensors to maintain the pressure balance in the systems and prevent the flooding of the evaporators. The complexity of the control system results in increased system cost, size and weight, and decreased system reliability and endurance.
It would therefore be desirable to have a heat transfer device that has the simplicity, reliability and cost advantages of passive devices such as heat pipes, loop heat pipes and capillary pumped loops, and the high heat flux and large heat carrying capacities of actively pumped and pump assisted devices.